Radio-frequency filters based on BAW resonators are of great interest for many RF applications. There are two concepts for BAW resonators, on the one hand the so-called thin-film BAW resonator (FBAR), and so-called solidly mounted resonators (SMRs). Thin-film BAW resonators include a membrane on which a layer sequence consisting of a lower electrode, a piezoelectric layer and an upper electrode is arranged. The acoustic resonator is formed by reflection at the upper side and at the lower side of the membrane. In the alternative concept of solidly mounted resonators, an SMR includes a substrate, such as, for example, a silicon substrate, on which the layer sequence consisting of the lower electrode, the piezoelectric layer and the upper electrode is arranged. In order to keep the acoustic waves in the active region in this design, a so-called acoustic mirror is required. It is located between the active layers, i.e. the two electrodes and the piezoelectric layer, and the substrate.
FIG. 2 shows an example of a solidly mounted resonator having a patterned mirror known from the prior art. A substrate 300 is shown, at the upper surface 304 of which an oxide layer 324 is deposited into which a trench or recess 326 is introduced. Further intermediate layers may be provided between the oxide layer 324 and the substrate 300. The acoustic mirror including a layer sequence comprising a first layer 306a1 having a high acoustic impedance, a layer 306b having a low acoustic impedance and a layer 306a2 having a high acoustic impedance is formed in the trench 326. On the surface of the resulting pattern, an insulation layer 308 on at least parts of which the lower electrode 310 is formed is deposited. The portion of the insulation layer 308 not covered by the lower electrode 310 is covered by another insulation layer 328. On the insulation layer 328 and on the lower electrode 310, the piezoelectric layer 312 on parts of the surface of which in turn the upper electrode 318 is formed is formed. The portions of the piezoelectric layer 312 not covered by the upper electrode 318 and parts of the upper electrode 318 are covered by the passivation layer 314. The overlapping regions of the lower electrode 310, the piezoelectric layer 312 and the upper electrode 318 define the BAW resonator 322.
In both concepts for BAW resonators, the deposition of a piezoelectric layer on a patterned bottom electrode is required, the bottom electrode being exemplarily made of a metal. A uniform deposition is thus of crucial importance for the quality of the resonator. To allow a uniform deposition, it is desired to have a highly planar waver surface, i.e. in particular a bottom electrode having a highly planar surface, before depositing the piezoelectric layer. Thus, growth edges in the piezoelectric layer are avoided and a high quality of the piezo material is made possible. This results in a high electro-mechanical coupling and a high acoustic quality. In addition, subsequent process steps are made easier by a planar surface. This particularly applies to patterning the upper electrode.
In methods known so far, this problem is solved as follows. In order to ensure a planar surface before the piezo deposition, the following steps are taken in processes known so far. At first, the bottom electrode is patterned, wherein the bottom electrode exemplarily is made of a metallic material and the patterning takes place through an etching process. After patterning the bottom electrode, a dielectric planarization layer (exemplarily made of a silicon oxide) having a thickness of roughly the step height after etching the bottom electrode is deposited. This planarization layer is subsequently opened in the region of the bottom electrode so that a rib having a width of exemplarily 1 μm remains at the edge of the bottom electrode. The width of this rib, however, depends on the selection of the etch mask used. In a subsequent oxide CMP (chemical mechanical polishing) step, this rib is polished off, which leaves a planar surface.
This method according to the prior art is problematic in that the oxide CMP step affects the surface of the bottom electrode. This results in a so-called “dishing” effect, a local variation of the electrode thickness. This also results in local variations of the piezo layer thickness deposited on the bottom electrode. Since the resonant frequency of the BAW resonator, except for the material of the piezo layer, is basically given by the layer thickness of the piezo layer, the local variation of the electrode thickness results in a widening of the series resonance of the BAW resonator, resulting in a considerable decrease in the series quality of the BAW resonator.
It would therefore be advantageous to provide a method allowing an improved manufacturing of a patterned bottom electrode in a piezoelectric device.